The present invention relates generally to batch distillation, and in particular to batch distillation processes and apparati to separate multi-component mixtures containing three or more components.
New, high-value specialty chemicals and pharmaceutical drugs are continually introduced in the market. Generally, these chemicals are separated through batch distillation. Batch distillation, as against continuous distillation, is used because it provides operational flexibility. It is not uncommon to use the same batch distillation equipment for a large number of products. In batch distillation, one distillation column is used to separate a multi-component mixture into several product streams. On the other hand, a continuous distillation system uses a number of distillation columns. The operating flexibility and equipment cost make a batch distillation column quite attractive for numerous distillation applications. It is well known, however, that batch distillation requires much more heat duty than continuous distillation (comparative energy consumption in batch and continuous distillation, O. Oppenheimer and E. SØrensen, Computers Chem. Engng, Vol 21, Suppl., pp S529-S534, 1997). An objective of the present invention is to reduce the energy consumption of batch distillation, reduce the time taken to distill a batch, and provide more choices for operating modes.
For the distillation of a multi-component mixture, both continuous and batch distillations are used. Generally when large quantities are to be distilled, continuous feed distillation is used, otherwise batch distillation is preferred.
The details of continuous multi-component feed distillation column arrangements can be found in U.S. Pat. Nos. 5,970,742 and 6,106,674, both assigned to Air Products and Chemicals, Inc. Consider the separation of a ternary mixture ABC having components A, B and C into three product streams each enriched in one of the components. A is the volatile component, C is the heavy component and B is of intermediate volatility. The continuous feed distillation schemes use two distillation columns. U.S. Pat. No. 5,970,742 discusses five well-known schemes: direct sequence, indirect sequence, side rectifier, side stripper and thermally coupled columns. These are described in FIGS. 1 through 5 of the ""742 Patent. This patent also describes some new continuous feed distillation schemes for multi-component feed distillation. In the direct and indirect sequences, feed is continually fed to a distillation column, a product stream is produced from one end of this column and a mixture from the other end is sent to the other column for further distillation. Each distillation column has a reboiler and a condenser. In thermally coupled column systems, fewer reboilers and condensers are used. This is achieved by having a two-way communication between the two distillation columns. In a two-way communication mode, when a vapor stream is sent from one column to another column, a return liquid stream is implemented between the two distillation columns. The side rectifier, side stripper and thermally coupled columns use thermal coupling to not only reduce the total number of reboilers and condensers in a continuous multi-component feed distillation but also to reduce the total heat demand for distillation.
Recently, cost reduction for continuous multi-component feed distillation columns with thermal coupling was suggested by building two or more distillation column""s functions in a single shell column. Some of these configurations are also known as divided wall columns. U.S. Pat. No. 2,471,134 was the first to describe a fully coupled divided wall column arrangement. U.S. Pat. Nos. 3,844,898 and 3,959,085 describe concentric cylinders in lieu of side stripper type configurations to distill a continually fed multi-component feed stream. Some recent examples of divided wall or partitioned distillation columns are given in two U.S. Patents assigned to Air Products and Chemicals, Inc.; namely U.S. Pat. Nos. 6,240,744 and 6,250,106. Two recent publications describe continuous multi-component feed distillation columns with partitions and multiple reboilers and condensers in detail (xe2x80x9cMore Operable Fully Thermally Coupled Distillation Column Configurations for Multi-component Distillationxe2x80x9d R. Agrawal, Trans. IChemE, Vol 77, Part A, pp 543-553, 1999; and xe2x80x9cMulti-component Distillation Columns with Partitions and Multiple Reboilers and Condensersxe2x80x9d, R. Agrawal, Ind. Eng. Chem. Res., Vol. 40, pp. 4258-4266, 2001).
Besides the continuous feed distillations, batch distillations are also used to separate multi-component mixtures. A recent review article provides a good survey of the state of the art (xe2x80x9cNew Era in Batch Distillation: Computer Aided Analysis, Optimal Design and Controlxe2x80x9d, K. J. Kim and U. M. Diwekar, Reviews in Chemical Engineering, Vol 17, pp 111-164, 2001). A conventional batch distillation column 110 is shown in FIG. 1. It consists of a reboiler 130 at the bottom and a condenser 160 at the top. The distillation column 110 includes devices that promote vapor-liquid contact for mass transfer and provide separation stages. Structured packing, dumped packing and several types of trays are used for this purpose. In a typical process, the multi-component mixture is charged either in the bottom sump of the column 110 or in the reboiler 130. Then heat is added to the reboiler and vapor ascends the column. The vapor is condensed in the condenser 160 through heat removal and collects in the reflux drum 162. Initially the condensed liquid is sent through line 166 to the distillation column and no product is collected from line 170. This operation is continued until the liquid holdup in both the column and in the reflux drum is met. After that the apparatus is run at total reflux until the acceptable purity of the volatile component A is achieved in the reflux drum. At that point, volatile product is withdrawn through line 170 according to a prescribed mode. The three modes often used are (i) constant reflux ratio, (ii) variable reflux ratio, and (iii) optimal reflux ratio. Reflux ratio is defined as the ratio of liquid flow rate in line 166 to the product flow rate in line 170. In variable reflux ratio mode, the reflux ratio is varied to keep the product composition constant. In optimal reflux ratio mode, reflux ratio is constantly changed to meet an objective function; e.g., to minimize the batch time for distillation. Generally, in constant reflux ratio and optimal reflux ratio modes, the composition of product in line 170 varies with time. When the product is collected in a storage vessel, however, the average composition meets the required specification. After the volatile component has been collected, a slop cut containing other components is collected through line 170. This is done until the desired purity of the intermediate volatility component is achieved. Then this product is collected through line 170 according to a prescribed mode. The slop cut is generally recycled to the distillation system in the next batch. The distillation is continued until all the desired product streams have been collected from the top reflux drum. In the end, the heavy component is recovered from the sump or the reboiler through line 140. When the batch distillation is run in this mode, the distillation column arrangement is known as xe2x80x9ctraditional batch column,xe2x80x9d xe2x80x9cconventional batch distillation,xe2x80x9d xe2x80x9cordinary column,xe2x80x9d or xe2x80x9cbatch rectifier.xe2x80x9d
In recent times, other batch distillation configurations have been suggested. These are also discussed in Chapter 9, pages 417-420, of the textbook by Doherty and Malone (Conceptual Design of Distillation Systems, M. F. Doherty and M. F. Malone, McGraw Hill, 2001). In a batch stripper case, most of the multi-component mixture is initially charged at the top of the column, such as in the reflux drum 162. The column is started by having some of the multi-component mixture charged to the bottom reboiler 130 and heat is provided to start the batch operation. No product is initially withdrawn. The column is operated until the desired purity of the heavy component is achieved in the column bottom sump. Then the heavy component is withdrawn in line 140 according to a prescribed mode of operation: constant reboil ratio, variable reboil ratio and optimal reboil ratio. The ratio of vapor flow rate in line 132 to the product flow rate in line 140 is called reboil ratio. The rest of the operating procedure is similar to batch rectifier with the difference that product streams of successively higher volatilities are collected from line 140.
Another novel batch distillation configuration uses a middle vessel column. In this arrangement, a multi-component mixture is initially charged to a middle vessel. Liquid from this middle vessel is fed to an intermediate location of the distillation column. The liquid from the tray above this feed location is withdrawn from the column and sent to the middle vessel. As a result, during a batch campaign, the middle vessel never gets empty. As the distillation progresses, the volatile component is withdrawn from the top of the distillation column, and the heavy component from the bottom of the column and the component of intermediate volatility accumulate in the middle vessel. Several operating strategies for the middle vessel column can be found in Hasebe, et. al. (xe2x80x9cSimultaneous Separation of Light and Heavy Impurities by a Complex Batch Distillation Columnxe2x80x9d, Journal of Chemical Engineering of Japan, Vol. 29, pp. 1000-1006, 1996) and the earlier mentioned review article by Kim and Diwekar. A pilot plant study of the middle vessel column using binary mixture is given by Barolo, et. al. (xe2x80x9cRunning Batch Distillation in a Column With a Middle Vesselxe2x80x9d, Ind. Eng. Chem. Res., Vol. 35, pp. 4612-4618, 1996).
Recently, Phimister and Seider have described a semicontinuous distillation using a middle vessel column arrangement (xe2x80x9cBridge the Gap With Semicontinuous Distillationxe2x80x9d, Chemical Engineering Progress, pp. 72-78, August 2001). In this arrangement, the multi-component mixture is initially charged to the middle vessel, and the distillate from the top of the column, concentrated in volatile component, is removed continuously, although in diminishing amounts. The column bottoms, consisting of heavy component, is also removed continuously in diminishing quantities. During this time interval, the middle vessel becomes concentrated in the intermediate volatility component. At this point, the middle vessel is nearly emptied and is then charged with the multi-component mixture. The whole process is then repeated.
The use of middle vessel column for batch extractive distillation is also suggested (for e.g., see earlier referenced book of Doherty and Malone and also the paper by Phimister and Seider). In this case, a heavy extractive solvent is added near the top of the column, above the feed point from the middle vessel, to break an azeotrope. The extractive solvent collects in the sump/reboiler of the column and is recycled. Phimister and Seider provide an example to distill an acetone/methanol mixture with water as an extractive solvent.
It is clear from the discussion that interest in batch distillation is currently on a rise. The reason for this is that a batch distillation is flexible; it can handle low throughput and requires low investment. This makes it quite attractive for fine/specialty and pharmaceutical chemicals.
A continuous feed distillation process runs at a steady state with a multi-component feed being fed at a constant rate to the distillation column system. Once the columns achieve steady state, there are no transients and the concentration profiles along the height of a distillation column do not change with time. The design of this column system is often optimized to perform one separation. As against the continuous feed distillation, in a batch distillation the multi-component mixture is not continuously charged at a fixed rate to the distillation column. It is generally charged at the start of a batch campaign. Sometimes it is charged at certain intervals of time. During a campaign, the column is always in transition; i.e., the concentration profiles along the height of the distillation column change with time. Quite often the holdups in certain locations also change. This is also true for so-called semi continuous distillation. Phimister and Seider""s paper clearly shows the variation with time in holdups and concentrations at several locations of a semicontinuous distillation column.
The present invention is a process for the separation of a multi-component mixture containing at least three major constituent components of different relative volatilities into product streams that are enriched in one of the major constituent components by batch distillation in a distillation column system. Therefore, one aspect of the present invention is an apparatus for (a) charging the multi-component mixture containing at least three major components to a distillation column system having at least three distillation zones, wherein one of the distillation zones is in fluid communication with the vapor and liquid flows of the at least two other distillation zones; (b) distilling the multi-component mixture within the at least three distillation zones such that distillation is conducted for at least a period of time without any addition of the multi-component mixture to the distillation column system; (c) collecting a light product stream enriched in the light component from the top of one distillation zone; (d) collecting a heavy product stream enriched in the heavy component from the bottom of a different distillation zone from step (c); and (e) collecting an intermediate volatility stream enriched in the medium component from a third distillation zone which is different from the distillation zones of steps (c) and (d).
According to one preferred embodiment of the present invention, the process comprises the steps of charging a multi-component mixture containing at least three major components to a distillation column; distilling the multi-component mixture within at least two distillation zones within the distillation column; providing a first portion of the distilled stream from the location where the two distillation zones meet, to a third distillation zone; collecting a light product stream enriched in the light component from the top of the distillation column; collecting an intermediate volatility stream enriched in the medium component from the third distillation zone; and collecting a heavy product stream enriched in the heavy component from the bottom of the distillation column.
Another aspect of the present invention includes a process comprising charging the multi-component mixture containing three or more major constituent components to a first distillation column; establishing a two-way communication between an intermediate section of the first distillation column and a first location of a second distillation column by feeding at least a portion of the vapor or liquid stream exiting from the intermediate section of the first distillation column to the first location of the second distillation column and in return withdrawing a stream of the opposite phase from the first location of the second distillation column and feeding it to the intermediate section of the first distillation column; and removing a first stream from the second distillation column, recycling a first part of the first stream back to the first distillation column and removing a second part of the first stream as a product stream from the distillation column system.
Still another aspect of the present invention includes a batch distillation column system for distillation of a multi-component mixture containing at least three major constituent components of different relative volatilities into product streams that are enriched in one of the major constituent components. The system comprises a first distillation column having a top and a bottom and at least two distillation zones disposed therein; a second distillation column having a top and bottom, the top of the second distillation column in two-way communication with the first distillation column at a point between the two distillation zones of the first distillation column; a reflux drum which receives condensed vapor from the top of the first distillation column; a first reboiler in communication with the bottom of the first distillation column; and a second reboiler in communication with the bottom of the second distillation column.
Still yet another embodiment includes process for the separation of a multi-component mixture containing at least three major constituent components of different relative volatilities into product streams that are enriched in one of the major constituent components by distillation in a distillation column system containing at least two distillation columns. This process comprises charging the multi-component mixture containing three or more major constituent components to a first distillation column; causing distillation to occur within the first distillation column until a majority of the heaviest component is below an intermediate section of the first distillation column; establishing two-way communication between the intermediate section of the first distillation column and the top of a second distillation column by feeding at least a portion of the liquid stream exiting from the intermediate section of the first distillation column to the top of the second distillation column and in return withdrawing a stream of vapor from the top of the second distillation column and passing the withdrawn stream back to the intermediate section of the first distillation column; condensing a vapor stream from the top of the first distillation column and collecting the resultant liquid in a reflux drum; returning at least part of the liquid collected in the reflux drum in the previous step to the top of the first distillation column; removing a first product stream from the reflux drum after an acceptable concentration of the most volatile component is reached in the reflux drum; removing a second product stream from the bottom of the second distillation column after an acceptable concentration of the intermediate volatile component is reached; and removing a third product stream from the bottom of the first distillation column after an acceptable concentration of the heaviest volatile component is reached.
Still yet another embodiment includes a batch distillation column system for distillation of a multi-component mixture containing at least three major constituent components of different relative volatilities into product streams that are enriched in one of the major constituent components comprising a distillation column having a top and a bottom and at least three distillation zones disposed therein, the distillation column having an upper and lower region, the lower region having a vertical separating element which defines the first and third distillation zones, the upper region defining the second distillation zone; a reflux drum at the top of the distillation column; a first reboiler at the bottom of the first distillation zone; and a second reboiler at the bottom of the third distillation zone.
An additional system embodiment in accordance with the present invention includes a batch distillation column system for distillation of a multi-component mixture containing at least three major constituent components of different relative volatilities into product streams that are enriched in one of the major constituent components comprising a first distillation column having a top and a bottom and at least two distillation zones disposed therein; a second distillation column having a top and bottom, the bottom of the second distillation column in two-way communication with the first distillation column at a point between the two distillation zones of the first distillation column; a reflux drum in fluid communication with the first distillation column; a first reboiler in fluid communication with the first distillation column; and a second reflux drum in fluid communication with the second distillation column.
Still another embodiment includes a process for the separation of a multi-component mixture containing at least three major constituent components of different relative volatilities into product streams that are enriched in one of the major constituent components by distillation in a distillation column system containing at least two distillation columns comprising charging at least a portion of the multi-component mixture containing three or more major constituent components to the first reflux drum located at the top of the first distillation column; causing distillation to occur within the first distillation column until a majority of the light component is above an intermediate section of the first distillation column; establishing a two-way communication between an intermediate section of the first distillation column and the bottom of a second distillation column by feeding at least a portion of the vapor stream exiting from the intermediate section of the first distillation column to the bottom of the second distillation column and in return withdrawing a stream of liquid from the bottom of the second distillation column and passing the withdrawn stream back to the intermediate section of the first distillation column; condensing a vapor stream from the top of the second distillation column and collecting the resultant liquid in a second reflux drum; returning at least portion of the liquid collected in the second reflux drum to the top of the second distillation column; removing a first product stream from the second reflux drum after an acceptable concentration of the intermediate volatile component is reached in the second reflux drum; removing a second product stream from the bottom of the first distillation column after an acceptable concentration of the heaviest volatile component is reached; and removing a third product stream from the first reflux drum at the top of the first distillation column after an acceptable concentration of the lightest volatile component is reached.
Still yet additional embodiments consistent with the present invention are included and are discussed in more detail below.